An understanding of sensory system development and the cellular mechanisms responsible for environmental influences on perceptual ontogeny is of fundamental concern to biology and psychology alike. Continued advancement will require increased knowledge regarding: a) the times and patterns in which the nervous system becomes responsive to stimuli impinging from the external environment; b) ontogenetic changes in the neuronal coding of sensory information; c) relationships between structural and functional indices of neuronal ontogeny; and d) how variations in the integrity and activity of afferents influence the ontogeny of neuronal elements. In this proposal we have outlined the continuation of a research program in which these problems are addressed in one "model system", the brain stem auditory nuclei of the avian embryo. During this grant period we have proposed to continue our ongoing neurophysiological investigations: a) of functional synaptogenesis of the chick brain stem auditory nuclei maintained in vitro; and b) of the in vivo ontogeny of acoustic coding by neurons in the eighth nerve, n. magnocellularis and n. laminaris (NL). In addition, the ontogeny of dendritic and synaptic morphology in NL neurons will be examined by quantitative Golgi and EM methods, and the influence of afferent innervation on localized dendritic development and cellular lamination will be directly assessed by blocking the ingrowth of normal afferents to one dendritic surface of NL. These investigations will increase our understanding of the ontogeny of sensory information processing, ontogenetic gradients in neural tissue, structure-function relationships in brain development, and the influence of afferents on neuronal development.